docs: def open(self, filename, mode='r', **kwargs):
          '''
          Open the file and return a file-like object.
  
          :param str filename: The storage root-relative filename
          :param str mode: The open mode (``(r|w)b?``)
          :raises FileNotFound: If trying to read a file that does not exists
          '''
          if 'r' in mode and not self.backend.exists(filename):
              raise FileNotFound(filename)
          return self.backend.open(filename, mode, **kwargs)